The Fundamentals of The basics Latching Solenoids: A Comprehensive Guide

Part 1 : What is a latching solenoid?

A latching solenoid is an electromechanical device that uses magnetic force generated by an electric current to maintain a set position without the need for continuous application of power. A latching solenoid (also called a bistable solenoid) has two standard positions: the plunger is fully extended when de-energized, and the plunger is held in place by a permanent magnet when de-energized.

Part 2 : Construction of a latching solenoid

Electromagnetic part: It is mainly composed of coils. When current passes through the coils, a magnetic field is generated. This is the core part that provides driving force.

Mechanical part: usually includes a plunger or armature. The plunger is affected by the magnetic field of the coil and can move linearly or rotate. In some designs, there are also components such as return springs to help the plunger return to its original position when power is removed.

Part 3 : How does a latching solenoid work?

Locking Mechanism: When a current pulse is applied to the solenoid coil, the resulting magnetic field attracts the plunger, causing it to move and lock into a specific position. This locking action can be used to hold a mechanical part in place, such as keeping a valve open or closed, or locking a switch in a specific state.

Unlocking mechanism: To release the latch, another current pulse with a different polarity or a specific control signal is usually required. This causes a change in the magnetic field, moving the plunger back to its original position, thus releasing the latch.

The electrical polarity is critical to the proper operation of a latching solenoid. When current flows in one direction, the coil field becomes energized, increasing the pull of the permanent magnet. The pull attracts the armature to the stationary pole. Once the armature is in full contact with the pole, the permanent magnet holds it in the latched position without further input of electrical energy. Sending current in the opposite direction through the coil field cancels the magnet's attraction and releases the plunger from the latched position.

Part 4 : Types of Latching Solenoids

4.1 Classification by working principle

• Permanent magnet latching solenoids: These solenoids use a combination of a permanent magnet and a solenoid coil. When a short pulse of current is applied to the coil, the permanent magnet receives a small magnetic flux, causing the plunger to move to a fixed pole. After the current is cut off, the permanent magnet holds the armature in the latched position.
• Residual Latching Solenoids: The internal design of these solenoids enhances their inherent residual magnetism. When energized, the magnetism of the coil enables the solenoid to latch. A reverse polarity current is required to unlock the solenoid until power is re-applied.

4.2 Classification by structure

• Open Frame Latching Solenoids: They have a compact design and low-cost construction, providing flexibility in a variety of applications. The open frame construction allows for easy observation and maintenance of internal components.
• Compact Latching Solenoids: These solenoids are fully potted for protection. They are suitable for applications where space is limited and a high degree of protection is required, such as certain miniature electronic devices.

4.3 Classification by application scenario

• Heavy-duty latching solenoids: Designed to handle high strength and loads, they are often used in industrial equipment, construction machinery, and other fields. For example, they can be used to control large valves in hydraulic systems or locking mechanisms for heavy machinery components.
• Miniature latching solenoids: Small in size, they are suitable for space-constrained applications such as portable electronic devices, medical devices with miniature components, and some precision instruments.
• Intrinsically Safe Latching Solenoids: These solenoids are designed for use in hazardous environments, such as areas with flammable gases or explosive substances. They feature high-speed "extend" operation and stringent safety standards to protect against sparks and other ignition sources.
• Latching solenoids with bursting pin: Designed for fire extinguishing systems, they need to generate a large force to penetrate the bursting disc and release the extinguishing agent or other substances.

4.4 Other special types

• Bidirectional DC Latching Solenoid: Can be controlled to latch in both directions, compact design, low power consumption, suitable for battery-powered applications requiring bidirectional control.
• Three-position latching solenoids: These solenoids have three positions and are usually spring-centered and latch into the end positions. They can provide more control states and are used in some applications that require more precise position control.
• Manual reset latching solenoids: They do not require constant power to hold their position and can be released quickly when activated. They will remain in the released position until manually reset, which is suitable for some applications that require manual intervention and control.

Part 5 : Key Features :

5.1 Electricity - Consumption - Freehold

One of the most notable features of a self-locking solenoid is its ability to remain in either end position with zero power consumption. Once the solenoid is actuated to achieve the desired locked state, no current is required to maintain that position. This is in stark contrast to traditional solenoids, which often require a continuous supply of power to hold a part in place. This feature of self-locking solenoids is highly advantageous for applications where energy conservation is critical, such as battery-powered equipment or large industrial operations designed to reduce electricity costs.

5.2 Zero power dissipation (heat) in hold state

In the hold state, the latching solenoid consumes zero power, which means no heat is generated. Since heat generation is directly related to power consumption according to Joule's law (\(P\) = I^{2}R\), where \(P\) is power, \(I\) is current, and \(R\) is resistance), the absence of current flow in the hold state ensures that no heat is generated. This is very useful in applications where overheating could cause problems, such as in sensitive electronics or environments with strict temperature control requirements. It also helps to extend the life of the solenoid and surrounding components, as overheating can cause material degradation and component failure.

5.3 No radiated electrical noise in hold state

Another significant advantage of latching solenoids is the absence of radiated electrical noise when in the holding state. When the solenoid is energized, the current flowing creates an electromagnetic field that can interfere with other nearby electronic devices. However, since latching solenoids do not draw current when holding their position, there is no such source of radiated electrical noise. This makes them ideal for use in applications where electromagnetic compatibility (EMC) is a concern, such as medical equipment, communications equipment, and high-precision measuring instruments.

5.4 Reliable termination - no power to hold position

Latching solenoids provide increased reliability by being able to maintain an end position in the absence of power. In the event of a power outage or electrical system failure, the solenoid’s latched state will remain unchanged. This is critical in applications where maintaining a specific mechanical position is critical to safety or proper functioning of the system. For example, in some industrial machinery, latching solenoids are used to hold valves in a specific position, and the solenoid’s ability to maintain this position in the absence of power ensures that the process can be safely paused or continued without interruption due to power-related issues.

5.5 Latch Solenoid Advantage

• Power saving: Compared with other solenoids that need to maintain continuous current to hold position, self-locking solenoids only need a brief pulse of current to lock or unlock. Once locked, they can hold the position without consuming additional power, which can save energy in some applications.
• Reliability: Relatively simple structure, stable locking performance, reasonable design and manufacturing, long service life and high reliability.
• Fast response speed: Latching solenoids can usually achieve fast locking and unlocking actions, meeting the requirements of applications that require fast switching or locking.

Part 6 : Applications of Latching Solenoids

Part 7 : Troubleshooting Latching Solenoids

7.1 Solenoid valve cannot be locked

Electrical problems

Power supply problem: Check that the power supply is providing the correct voltage. Use a multimeter to measure the voltage at the solenoid terminals. If the voltage is too low or too high, it may prevent the solenoid from latching. Make sure the power supply is functioning properly and that there are no loose connections or damaged wires in the power circuit.

Faulty coil: A burned or damaged coil may be one of the causes. Inspect the coil for signs of discoloration, melting, or physical damage. Use an ohmmeter to measure the resistance of the coil. If the resistance is significantly different from the value specified in the solenoid data sheet, the coil may be faulty and need to be replaced.

7.2 Mechanical problems

Stuck Plunger: The plunger may be stuck due to dirt, debris, or misalignment. Carefully clean the plunger and surrounding area and inspect for signs of wear or damage. Make sure the plunger moves freely in its hole. If misalignment exists, adjust the position of the solenoid or inspect the mounting bracket.

Spring problem: A weak or broken return spring can prevent the plunger from locking properly. Check the spring for any signs of deformation, breakage, or loss of elasticity. Replace the spring if necessary.

7.3 Solenoid valve cannot be unlocked

Electrical problems

Control signal problem: Check that the control circuit is sending the correct unlock signal. This may involve a problem with the microcontroller, relay, or other components in the control system. Use an oscilloscope or logic probe to verify the signal waveform and timing. Make sure the signal has the correct polarity and voltage level.

Reverse Polarity Protection: Some latching solenoids have reverse polarity protection diodes. If these diodes fail, they prevent the solenoid from receiving the correct unlocking signal. Inspect the diodes for any signs of damage and test them with a diode tester. If the diodes fail, replace them.

7.4 Mechanical problems

Debris or contamination: Similar to the lockout issue, dirt or debris can get stuck in the mechanism and prevent the plunger from moving back to the unlocked position. Clean the solenoid thoroughly and remove any foreign material.

Worn or damaged components: Over time, the mechanical components of the solenoid valve may wear. Inspect the plunger, latch mechanism, and any other moving parts for signs of excessive wear, deformation, or damage. Replace worn or damaged components to restore normal operation.

7.5 Solenoid valve intermittent operation

Electrical problems

Loose connections: Check all electrical connections in the solenoid circuit, including terminals, wires, and connectors. Loose connections can cause the solenoid to lose power, resulting in erratic operation. Tighten all connections and ensure they are secure.

Electromagnetic Interference (EMI): External electromagnetic fields can interfere with the operation of the solenoid valve. This is more likely to occur in an environment with a lot of electrical equipment or high-frequency signals. Use shielded cables for solenoid valve wiring and ensure that the solenoid valve is properly grounded to reduce EMI.

7.6 Environmental factors

Temperature Effects: Extreme temperatures can affect the performance of a solenoid. Low temperatures can cause the coil resistance to increase, while high temperatures can cause the coil to expand and affect the magnetic field. If the solenoid is operating in an extreme temperature environment, consider using a solenoid with a wider temperature range or providing additional thermal protection.

Vibration and shock: Excessive vibration or shock can cause the solenoid assembly to loosen or misalign, resulting in intermittent operation. Secure the solenoid properly and use shock-absorbing mounts if necessary.